Printed electronic components and devices have been, for the most part, demonstrated on non-absorbing substrates. This way, the physical properties of the printed (and cured) layer are principally determined by the material itself and e.g. substrate surface modifications only target desired wetting behaviour during printing. Contrary to e.g. inkjet photopapers or transparency sheets, these printing substrates require the complete evaporation of the solvent/dispersant rather than drying by absorption. In addition to the removal of the solvent, most particle based inks/pastes typically require a curing step to remove a particle encapsulating material to further enable physical contact between the particles. The encapsulating material is often tightly bonded to the particle cores to provide a stable dispersion by preventing particle aggregation. Therefore, the curing temperature required for the efficient removal of the encapsulant is often challenged by the thermal tolerance of the substrate. This is a problem frequently encountered when attempting to form metallic conductors on flexible substrates by means of printing a dispersion of encapsulated metallic nanoparticles followed by thermal curing to remove the encapsulant to enable sintering. Thereupon, various alternative sintering methods such as laser sintering, pulsed high-energy light sintering, microwave sintering and electrical sintering have been developed. Electrical sintering, for example, is discussed in our earlier patent application publications WO 2008/009779, EP 2001272, EP 2001053, EP 2001273 and EP 2003678.
All known sintering methods have their advantages but they also suffer from certain disadvantages.
The printing of devices and circuit components is still at a technology development stage: process related limitations (e.g. the minimum gap spacing between printed conductors) as much as material limitations (e.g. the stability and mobility of organic semiconductors) persist regardless of the efforts of the scientific community and industry. Apparently, some pioneered printed electronics processing lines are capable of all-printing fully functional RFID tags, but these typically require expensive equipment and a controlled environment. More mature technologies such as screen printing of silver paste for antenna fabrication have proven competitive, though the integration of the silicon based microchip, or other discrete components, with the printed circuitry (antenna) is a costly process stage. A typical method used for forming the interconnection is flip-chip bonding (pick-and-place) although other methods such as fluidic and vibratory assembly have been introduced. The generic problem with interconnecting components to the printed circuitry is how to deliver a discrete component to the precise location (aligned to the printed contact pads) in a high throughput manner. An additional challenge related to flip-chip bonding to (nanoparticle ink) inkjet printed and sintered wiring is the electrically insulating layer that tends to form on the top surface of the sintered conductor.
Thus, there is a need for an efficient method for removing the encapsulating material from deposited particles to expose the particle cores in particular for the purposes of conductor forming, component interconnecting, forming sensor materials and promoting the sintering of metal oxide and semiconductor particles. A process method and apparatus, by which even large-scale circuits could be fabricated on low-cost substrates by drop-on-demand printing (digitally defined print pattern) and chip-on-demand placing (various different components place at digitally defined component locations with respect to the printed electrodes), would inspire much potential.